The present invention generally relates to an optical recording medium capable of recording, reproducing and erasing information with high density and mass by laser beams.
An optical disc as the optical recording medium is classified into a postscript type disc and a rewriting type disc in accordance with a recording thin film to be used. In the postscript type disc, a record reproducing operation can be effected. Once data are stored, the record cannot be erased so that the data cannot be stored again. In the rewriting type disc, data can be rewritten many times on the same data region.
In the postscript type disc, data are recorded on a recording thin film by the irradiation of laser lights. The data are reproduced by detection of the variation in the quantity of light reflected from the recording thin film with a photo diode. The recording thin film to be used here is a thin film made of a TeOx (0&lt;x&lt;2.0) component chiefly composed of Te and TeO.sub.2. The condition of the film is changed from an amorphous condition to a crystal condition by a rise in a film temperature caused by the irradiation of the laser lights. The recording operation of the data is effected with the use of the phenomenon. The recording thin film cannot be changed from the crystal condition to the amorphous condition. Although the record reproducing operation can be effected in the postscript disc, the rewriting (overwriting) operation cannot be effected.
The optical magnetic recording disc and the phase change recording disc are the main types of overwrite discs. In the optical magnetic recording disc, a magnetic thin film which is an amorphous alloy film composed of rare earth transition metal is used as a recording thin film. In the magnetic thin film, the film temperature rises, and coercive force reduces in a portion upon which the laser rays are irradiated. When a weak external magnetic field has been given to such a magnetic thin film, the portion is retained as a spot shaped magnetic area with magnetizing inversion being caused. The data are recorded by this use of the phenomenon. In the reproducing operation, the polarized face of the laser light is rotated by the magnetization direction. Laser lights of a power weaker than at the recording time are irradiated upon the magnetic thin film with the use of such phenomenon so as to detect the varied condition of the polarized face of the laser light. The detecting operation is effected with the use of an element called an analyzer which measures the change in the condition of the polarized face in the laser lights. As the condition can be restored to the original one by the provision of a strong magnetic field in the above described spot shaped magnetic area magnetically inverted, the overwriting operation can be effected.
The recording disc is normally orbicular in shape. It is used by the rotation thereof. In the phase change recording disc, the laser lights are throttled to approximately 1 .mu.m and are modulated in strength in accordance with the information being imparted to the disc, with the recording thin film being previously crystallized, the laser lights are irradiated. In this case, a peak power laser light irradiation portion is heated, fused to a fusing point or higher of the recording thin film, and thereafter, is quenched. An amorphous mark is formed on the recording thin film in this manner. The information is recorded with such a mark being provided as a recording mark.
The laser lights are likewise irradiated with respect to the record mark in the amorphous condition formed on the record thin film so as to effect the heating operation to a temperature, for easier crystallization, of the glass transition point (the fusing point or lower) or higher. It is retained for a given time so that the crystallizing operation is effected. In this manner, the recording mark, namely, the already written signal information, is erased. In the reproducing operation, variation in the reflection light quantity from the recording thin film is detected with the photo diode as in the above described postscript type disc.
In the phase change recording disk capable of recording, reproducing and overwriting operations, a chalcogen material such as Ge15Te81Sb2S2 or the like proposed by U.S. Pat. No. 3,530,441 to S. R. Ovshinsky and or a material with a chalcogen element such AS2 S3, As2 Se3, Sb2 Se3 or the like and elements of group V or group IV of the periodic table being combined are widely known as the recording thin film material.
As the recording thin film is repeatedly heated and cooled as described hereinabove, a protective layer composed of a dielectric superior in heat-proof property is normally provided on the top and bottom faces thereof. As the temperature rising, sudden cooling and slow cooling characteristics vary in the recording thin film by the thermal conduction characteristics of such a protective layer, the recording and erasing characteristics can be improved by the choice of the quality and the layer construction.
As the recording and erasing operations are repeated when the overwrite type disc by the phase change recording system is used, the number of the phase changes in the recording thin film is extremely increased, thus causing inconveniences in that the signal quality necessary to the information reading operation varies. The strict thermal operation of quickly heating the recording thin film to 4000.degree. C. or more, and thereafter, quickly effecting a cooling operation is repeated by the irradiation of the recording spot lights and the erasing spot lights composed of laser lights. The recording thin film itself and a protective layer provided on the top and bottom faces thereof are thermally damaged. Further, such protective layer is repeatedly expanded and contracted so as to effect a pulsating operation. The film thickness change of the recording thin film is caused so as to make the recording condition unequal.
In order to remove the above described inconveniences, it is proposed that a so-called thick film structure of mechanical strength be made higher by the smaller thermal capacity with the film thickness of the recording thin film being made thinner and the film thickness of the protective layer provided on the top, bottom faces thereof being made thicker. In the countermeasure, a phenomenon where the heat is confined in the protective layer is caused by many recording and erasing operations because of the thicker film structure. As a result, the signal quality is deteriorated because of the heat.